A relay node (RN) in a wireless communication system sits between one or more mobile stations (user equipment, or “UEs,” in 3GPP terminology) and a conventional base station in a wireless network, and is used to receive and re-transmit, i.e., forward, signals sent by the base station and intended for mobile stations, as well as to receive signals transmitted by the mobile stations, for forwarding to the wireless network. A number of UEs can be served by a single relay. One objective of the relay node is to enhance the radio access coverage in both the uplink (mobile station to base station transmissions) and downlink (base station to mobile station transmissions).
In the “Long-Term Evolution” (LTE) systems developed by the 3rd-Generation Partnership Project (3GPP), the specified architectures for relay node connectivity include a RN connected wirelessly to a “donor cell” of a “donor base station.” Base stations are known as eNodeB's, or eNB's, in 3GPP terminology, thus the donor eNB is referred to herein as a DeNB. The connection to the donor eNB is via a radio backhaul link. Simultaneously, one or more UEs may be connected to the RN, via the radio access link.
In LTE, the backhaul link (DeNB-RN link) and access link (RN-UE link) are called the Un and Uu interfaces respectively. FIG. 1 illustrates the connections among the various nodes in a network that includes relay nodes. In this straightforward configuration, UE 110 communicates (wirelessly) with RN 120 over the Uu interface, which is the access link. RN 120 in turn communicates (wirelessly) with eNB 130 over the Un interface, which is the backhaul link. eNB 130, in its turn, is connected to the Evolved Packet Core (EPC) 140, using the standard interfaces developed by 3GPP.
In practice, the RN may be fixed or movable. Both of these types of RN are wireless RN in that they receive and transmit wireless signals over the access and backhaul links. A “fixed” RN is stationary or immovable. For simplicity, the generic term RN or relay is used throughout this document. In some cases, a movable RN may be implemented as a standalone mobile relay, i.e., as a wireless device dedicated to operation as an RN. In other cases, a wireless terminal that is capable of providing end-user services, whether to a user or to a machine, such as in a machine-to-machine context, may also act as a relay. In some cases, a mobile relay may be deployed in a moving vehicle such as a bus, train, ferry, or the like, in which case it may primarily serve UEs that are aboard the movable vehicle. However, a moving mobile relay may also serve UEs in surrounding areas.